Galileo to Jupiter; Gathering Treasures on Its Loop-the-Loop Journey

Eberhart, Jonathan, Science News

Galileo to Jupiter

NASA planners devised Voyager 2's string of close encounters with Jupiter, Saturn, Uranus and Neptune to take advantage of a planetary alignment that occurs only once every 175 years or so. The complex trajectory of the upcoming Galileo mission to Jupiter, which will sweep the craft past Venus, Earth, an asteroid, Earth again and another asteroid, emerged largely as a consequence of the Challenger disaster.

Galileo, scheduled for liftoff aboard the shuttle Atlantis as early as Oct. 12, won't arrive at the solar system's largest planet for more than six years. When it does, the two-part craft -- an orbiter and a probe that will plunge into Jupiter's stormy atmosphere--should provide a wealth of information about the planet and its fascinating moons. But even before that, scientists expect Galileo to return troves of data as it loops through its 3.86-billion-kilometer tour of space on the way.

After the space shuttle Challenger disintegrated in flight on Jan. 28, 1986, NASA abandoned plans for an upper-stage rocket called the Centaur, intended for use aboard the shuttle. Centaur, fueled by touchy propellants, would have been by far the most powerful booster ever to send shuttle payloads out of their initial, Earth-circling orbits. One of the missions dependent on Centaur was Galileo, originally scheduled to depart Earth in May 1986.

Stuck without an adequate replacement booster, NASA redesigned the mission to fire Galileo using a conventionally fueled but less powerful rocket called the Inertial Upper Stage. To make up for the weaker substitute, Galileo will pick up the speed it needs to reach Jupiter by first heading in toward the sun and flying past Venus next February for a "gravity assist" that will also re-aim its trajectory outward. This will carry Galileo past Earth and onward to fly by an asteroid named Gaspra, making Gaspra the first object of its kind ever approached by a spacecraft. Then the vehicle will head back past Earth again, bend outward to go by a second, more distant asteroid known as Ida, and finally move on to enter the Jupiter-circling orbit that is its ultimate destination.

But Galileo's side trips en route promise more than just an intricate way to reach Jupiter on Dec. 7, 1995. The dates shown below may change if the launching is delayed, but mission officials have planned five separate encounters--each with its own research goals -- before Jupiter even enters the picture.

Feb. 9, 1990: Venus. The question of lightning on Venus remains unresolved and controversial, with scientists also disagreeing about whether such lightning might signal volcanic activity on the planet. Passing only 15,000 km away, Galileo will search for lightning flashes both by taking pictures of the cloud-shrouded world's night side and by listening for "whistlers," plasma-wave radio emissions associated with lightning.

Photos taken in ultraviolet sunlight reflected from the clouds will focus on small-scale motions of the atmosphere; infrared spectral images and measurements along the planet's edge will address recent questions about whether more water exists at high altitudes than previously believed.

Galileo will not send its Venus data to Earth until October 1990. At the time of encounter, its antennas will be variously covered by protective thermal sunshades or pointed in the wrong direction. For this reason, the data will remain stored on board, to be radioed back long after the spacecraft rushes around Venus.

Dec. 8, 1990: Earth. "It was not a foregone conclusion that Galileo would be allowed by NASA management to execute observation sequences during the Earth flybys," according to a written account of the mission plans by Science Data Team Chief Theodore C. Clarke of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and Satellite Working Group Chairman Fraser P. Fanale of the University of Hawaii in Honolulu. …

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